Although both indentured servitude and slavery were examples of significant human rights abuses and ways in which workers were exploited and mistreated, they were quite different legally.
A slave, by definition, was property. Legally, a slave had no more rights than a piece of furniture and was not, in legal documents, considered a human being. They were merely considered possessions. Slaves had no legal rights and could be bought and sold. Slaves had no say about whether they became slaves or not. They were kidnapped, conquered, or born into slavery. Although owners could free slaves, this was purely the choice of the owner, and manumission depended entirely on the owner's whim. However, in ancient Rome under certain circumstances, slaves could have the opportunity to buy their freedom.
Indentured servants, on the other hand, legally counted as human beings with certain rights. In theory, people voluntarily entered into contracts to become indentured servants, and these contracts had fixed durations, during which people were not free to seek other employment or to travel. Many other limitations in personal freedom might apply for the duration of the contracts.
In the United States, indentured servants agreed to a fixed term of labor in return for transportation to the US, room, board, and a "freedom package" payable on completion of the period of indenture, often including land, seeds, and domestic animals, which would give such workers a start as farmers. Although the system was subject to abuse, for many poor people in Europe, it provided a legitimate, if difficult, path to a new life in the New World.